Between 1850 and 1860 the black population of Illinois grew faster than the black population of Florida and Georgia and Alabama. The black population of Mississippi and Arkansas actually shrank. And whatever restrictions were placed on blacks in Illinois paled in comparison to those imposed on blacks in the South where they were also denied entry to states unless they were property, severly denied and restricted in freedom, and were expelled from some states.
That's a very sloppy use of statistics. If there were only 7 to 8 thousand blacks there in 1850 it only takes a small increase in absolute numbers to equal a large percentage increase.